Direct current (DC) Offset is a phenomenon that causes the excursion of the diaphragm (or cone) of a transducer (and of micro-speakers in particular) to manifest an asymmetric behaviour (the diaphragm moves more in one direction than the other). This DC offset may be caused, for example, by one or more of following three example factors:                1) As a result of problems in the mechanical design of the transducer or in its manufacturing process, the diaphragm of the transducer may not be centred with respect to the magnet and, therefore, even at rest, the diaphragm may be in a position that is not central to the magnets. This non-central position may lead to asymmetrical movements of the diaphragm regardless of the amplitude and/or frequency of the driving signal.        2) Differences in the air-load in the front and back cavity of the transducer may result in asymmetries in the movement of the diaphragm as the diaphragm may face different air resistances when moving in one direction compared to the other. This result may be caused by a poor or constrained acoustical design, or by flows in the manufacturing process of the transducer as well as any external factors (e.g. presence of water or a finger obstructing a front cavity of the transducer).        3) The typical asymmetry in the nonlinear characteristics of the electrical and mechanical parameters of the transducer also contribute to the DC offset. In particular, such nonlinear characteristics are the nonlinear behaviour of the force factor, the stiffness and the inductance of the transducer (respectively BI(x), Cms(x) and Le(x)). These nonlinearities may become particularly evident when the transducer is driven with large signals (that therefore can cause more excursion).        
Various studies focus on the causes and effects of DC offset, which is a complex phenomenon to analyse and model in detail. The factors listed above can all manifest in a single transducer with results that are difficult to accurately predict.
From the point of view of transducer protection applications, the presence of DC offset can be a problem if the transducer is driven close to an excursion limit of the transducer without considering that the diaphragm could be exceeding the safe excursion region when it moves in one direction due to the DC offset.
In typical transducer protection applications for the consumer electronics industry, the micro-transducer design and manufacturing process are fairly well controlled, so the main cause of DC offset in normal use cases that require protection is the third factor listed above. The focus of the embodiments described herein is therefore to address DC offset that manifests with large signals and that may not be constant.
FIG. 1 illustrates the DC offset profiles of three example speakers or transducers of the same type, model or design. As can be seen from this figure, the DC offset profile of the three speakers, while not identical, is of a similar shape. In particular, in all three profiles, a maximum DC offset is reached at around the same frequency, which in this example is around 1100 Hz. This frequency at which the maximum DC offset is reached is often the resonance frequency for a particular type of transducer. The maximum values of the DC offset are also the same/similar, in this example around −0.09 mm. This nonlinear profile may therefore be used to estimate the direct current (DC) offset caused by the third factor (problem 3 above) for a particular speaker/transducer type.
FIG. 2 illustrates measurements of DC offset for three different transducer types, models or designs. In this example, Speaker A is illustrated by line 201, Speaker B is illustrated by line 202 and Speaker C is illustrated by line 203.
As seen from FIG. 2, transducers of different types may have different DC offset profiles. Therefore, while it may be possible to draw conclusions across all transducer samples of a certain transducer type, the same results may not be valid for different transducer types.
A transducer protection system takes into account the effect of DC offset when limiting the excursion of the transducer. An accurate and reliable DC offset prediction model is however difficult to design and use in real applications because of the complexity of the phenomenon. A good nonlinear model of the speaker or transducer may be able to estimate the amount of DC offset, but it needs to either model online or know in advance the nonlinear behaviour of the transducer parameters. It may not be possible to run a model on resource-constrained devices because of limitations in Million Instructions per Second (“MIPS”)/memory, and/or because running the characterization process required to gather the parameters and data needed by the nonlinear model to be robust across a wide distribution of speakers or transducers could be impractical in certain applications.